In a system where radio signal bursts are present while having audio channel open, a common audio noise problem appears called “burst noise” or “time division multiplexing access (TDMA) noise”. This problem is particularly present on uplink paths where a microphone is the electrical source of audio. In fact, microphone signal is very low, around mV, and any noise above μV can be heard. The burst noise can be really annoying and advantageously has to be removed.
This burst noise exists in the audio channel due to two common coupling mechanisms used in radio transmissions:                A radio power amplifier consumes significant amount of current when transmitting while the current consumption is very low when not transmitting. This current consumption variation is creating a voltage variation on the battery supply for the entire product due to battery impedance. The audio portion of the system is powered by the same battery voltage. Consequently, depending on the power supply rejection ratio of the solution, this signal is partly injected into the audio path.        During radio transmission, a significant electromagnetic field is created on the product due to antenna. A relatively big part of this radio signal is going to be injected into the audio portion. Because the composition of audio system is based on transistors which are mostly non linear, the radio signal is rectified. Due to the rectification, radio signals inject direct current (DC) component into the audio path. As radio activity comprises signal bursts, the presence of the DC component is not continuous. This results in an audio noise coupled with the radio power amplifier activities.        
Thus, a radio activity comprising signal bursts is by itself creating noise on the audio path. This noise level is not constant in time:                GSM/GPRS/EDGE phones have the capability of operating in discontinuous transmission mode, also referred to as DTX, in order to reduce current consumed by the phone. This occurs when the local voice is not active. Under such conditions the burst noise will be discontinuous as well.        GSM/GPRS/EDGE phones have the capability of changing the power of the radio transmission. Then, the burst noise level will follow the power change of the radio.        Different coupling gains between radio portion and audio path are not static. In fact, the local environment of the phone affects the antenna and the radiation and/or receiving pattern. Then the radio power amplifier could consume variable amount of current and the portion of radio signal injected into an audio path can vary. This results in non-fixed coupling gains meaning a non-fixed level of burst noise.        
Several solutions are known to reduce burst noise. First, one solution is based on reducing coupling gains. The burst noise is created from the coupling of radio activity to the audio path. If this coupling gain is low enough, the resulting burst noise could be low enough not to be heard.
Reducing coupling gains involves the use of efficient power supply paths for audio to get high power supply rejection ratio. The battery impedance has to be low as well. The audio signal routed on a printed circuit board (PCB) needs to be protected, for instance routed by a pair and placed behind a shield. Some passive linear components have to be inserted at different locations to evacuate the radio signal from the audio lines.
This method has the disadvantage that the addition of passive components and efficient power supply paths has a cost. The routing of audio lines on PCB can be very painful. On some products, because of their size and mechanic arrangement, it is even not possible to remove completely the burst noise.
Second, another method of reducing burst noise is based on a burst noise filter. The burst noise by itself follows the signal bursts. In a GSM/GPRS/EDGE phone, the standard specifies that a transmit burst occurs once in a period of 4.615 ms in continuous transmission and most of time at the same location, i.e. the same time slot. Because of that, the burst noise will be a signal composed by a fundamental at 216.7 Hz (1/4.615 ms) and its harmonics. So, it is possible to build a filter which rejects only these frequencies resulting in the removing of burst noise.
However, this method also has a drawback, namely because of the filter insertion, the voice signal itself suffers attenuation on the rejected frequencies. For this reason, the voice quality can be significantly damaged.
Third, still another method is based on a noise suppressor. For different reasons phones embed a noise suppressor. The algorithm used in noise suppressors can be based on spectral subtraction technique. Its functioning principle is to estimate the noise spectrum in absence of speech activity and to subtract in frequency domain this estimated noise. As noise shape is quite constant compared to voice activity, the subtraction of it is still valid even during speech periods. This results in the cancellation of noise even with presence of a speech signal. In continuous transmission, the burst noise has also a quite constant shape and spectrum. Then, the generic noise suppressor will have cancellation ability for it, resulting in a reduction of burst noise.
However, as explained above, the burst noise has constant shape only in the continuous transmission phase. When a phone is operating in discontinuous transmission mode, the spectrum of the noise is no more constant and the noise suppressor is not be able to correctly estimate it, resulting in non efficient cancellation. Furthermore, GSM/GPRS/EDGE phones can adapt suddenly the power of their radio transmission resulting in a new spectrum composition of the burst noise. This drives also to a bad estimation of noise by the noise suppressor resulting in a suboptimal noise cancellation. So, the noise suppressor cannot take into account sudden spectral changes. For this reason, the noise suppressor does not work properly in systems based on GPRS (because several time slots can be used for transmission in one frame) or in some 3G systems (because of too complex and non-constant spectrum).
It is thus the object of the present invention to overcome the above-identified difficulties and disadvantages by proposing an improved solution for noise cancellation.